#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
 
#define LEN 100
 
static char input[LEN], hold[LEN];
static int i, j, cnt;
 
void eval();
 
int main(void)
{
 
	puts("Enter an infix expression:");
	fgets(input, LEN, stdin);
 
	eval();
	return 0;
}
 
void eval()
{
	int x = 0;
	if (input[i] == '(') i++; 
	if (input[i] == ')')
	{ 
		printf("%c", hold[--j]); 
		i++;
	}
	if (input[i] == '*' || input[i] == '+')
	{ hold[j++] = input[i]; i++; }
	while (isdigit(input[i]))
	{
		x = 10*x + (input[i++]-'0');
		printf("%d", x);
	}
	eval();
}

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
 
#define LEN 100
 
static char input[LEN];
static int i;
 
void eval();
 
int main(void)
{
	puts("Enter an infix expression:");
	fgets(input, LEN, stdin);
 
	eval();
 
	return 0;
}
 
void eval()
{
	char hold;
 
	if (input[i] == '(') i++;
	if (isdigit(input[i]))
	{ printf("%c", input[i]); i++;}
	if (input[i] == '*' || input[i] == '+')
	{ 
		hold = input[i];
		i++; eval(); 
		printf("%c", hold);
	}
	if (input[i] == ')') 
	{ i++; return; }
	eval();
}

int product(int  n);
int main(void)
{
char   buf[100];
int    num;
printf("Enter number for porduct : ");
fgets(buf, sizeof(buf), stdin);
num = atoi(buf);
num = product(num);
printf("num = %d\n", num);
printf("Press enter to end ....\n");
getchar();
return(0);
}
/* There are 3 steps in writing a recursion 
   1) assume it knows how to calcluate the 
      previsous value.
   2) write the current value by the 
      previsous step.
   3) Make a stop conditon.
*/
int product(int  n)
{
int  tmp;
 
 
/* step 3 */
if ( n < 2 )
   return(1);
 
 
/* step 1 */
tmp = product(n-1);
 
/* step 2 */
return(n * tmp);
 
}